A typical MOS transistor includes a source/drain region, a channel region disposed between the source region and the drain region, and a gate electrode disposed over the channel region. The resistance of the gate electrode can be closely related to the speed of the MOS transistor. Namely, the less the resistance of the gate electrode is, the higher the operation speed of the MOS transistor may become. Usually, the gate electrode is made of doped polysilicon and tungsten silicide which are sequentially stacked. Because the tungsten silicide can have lower resistance than the doped polysilicon, the resistance of the gate electrode can be lowered. As integrated circuits become more highly integrated, there may be a demand for materials having lower resistance than tungsten silicide.
It is known to provide a metal silicide, such as cobalt silicide, titanium silicide or nickel silicide to provide lower resistance. However, the metal silicide may be deteriorated by a gate thermal oxidation process performed after forming the gate electrode. The gate thermal oxidation process is a thermal process for curing both sidewalls (particularly, lower sidewalls adjacent to the substrate) and an active region surface, which may be damaged during an etching process for forming the gate electrode. The gate thermal oxidation process may adversely affect the characteristics of the metal silicide thereby increasing the resistance of the metal silicide.